This invention relates in general to the field of microelectronic device manufacturing, and more particularly to a method and system for evaluating photomask inspection tools.
Over the past several years, the performance of microelectronic devices fabricated on semiconductor substrates has rapidly and consistently improved. Processing speeds have increased dramatically as device technology nodes have shrunk. For the most part, the dramatic improvements in microelectronic device performance have resulted from the use of the same underlying manufacturing technology. For instance, manufacturing of a microprocessor device on a silicon substrate involves patterning several layers of features in the silicon and depositing metal, such as aluminum or copper, in the features. Microprocessor device patterns are created on a photomask which is then used to etch the patterns into a semiconductor substrate. Each layer of a microelectronic device may include a number of different features that interact with each other and with different layers of the device. The photomask generally must have precise device feature dimensions for accurate etching of a desired pattern onto a substrate.
To ensure the accuracy of the features on a photomask, photomasks are typically inspected before they are used to manufacture semiconductor devices. Photomask inspection tools perform automated inspections of photomasks through the use of algorithms that help identify faults or defects in the photomask patterns. A typical inspection of a photomask is likely to identify a number of faults since the manufacturing of a photomask is prone to some error. Once an inspection tool identifies faults, the faults are repaired by a repair tool and the repaired photomask is used for manufacturing semiconductor devices. Accurate inspection and repair of photomasks is an important process for microelectronic device manufacturing since a single undetected fault in a photomask can result in costly manufacturing errors.
Photomask inspection tools are typically qualified for a desired sensitivity, meaning the inspection tool""s ability to find a smallest defect. For instance, the Verimask, available from Dupont Photomasks, Inc., is commonly used in a photomask production environment as a daily qualification test vehicle for photomask inspection tools. An inspection tool""s sensitivity test typically involves the inspection with the tool of a sensitivity module, such as the Verithoro module, to determine the size of programmed defects that the inspection tool detects and/or fails to detect. Programmed defects are intentional faults included in a photomask to ensure that inspection of the photomask detects known faults down to a known sensitivity level. An evaluation of the sensitivity of a photomask inspection tool evaluates the tool""s ability to detect programmed defects of predetermined size, but fails to provide a complete evaluation of the inspection tool""s ability to provide accurate inspection results for typical device features of different sizes. For instance, inspection of some features within the sensitivity of an inspection tool may result in false fault detections.
An evaluation of an inspection tool""s sensitivity provides some indication of the size of the smallest programmed defect that the tool accurately detects but fails to completely assess how well the inspection tool is able to run. The robustness of an inspection tool, meaning the inspection tool""s ability to run in a production environment, is difficult to define based on the tool""s sensitivity, especially in light of the expanding number of features found on photomasks. As one example, optical proximation corrections (xe2x80x9cOPCxe2x80x9d) introduce optical corrections to the mask pattern to correct for refraction errors that occur when the mask is used to etch a substrate. As device feature dimensions shrink, the nature of corner roundings of OPC features has greater effects on the inspectivity of an inspection system due to the difference between the data and mask. Essentially the inspection tool thinks the difference is a defect, but the difference is actually a feature rounding. Therefore, the inspection system signals a false defect when it is unable to differentiate rounding.
Therefore a need has arisen for a method and apparatus which evaluates the robustness of a photomask inspection tool""s ability to inspect a photomask with best sensitivity.
A further need exists for a method and apparatus which evaluates the runability of a photomask inspection tool for locating false errors associated with photomask features at different technology nodes.
In accordance with the present invention, a method and apparatus is provided that substantially eliminates or reduces disadvantages and problems associated with previously developed and methods and apparatus for evaluating the effectiveness of photomask inspection tools. Plural device features are written on a photomask at plural technology nodes. The runability of the inspection tool is determined by the runable smallest technology node for the features at which no inspection false errors occur.
More specifically, a set of die simulates a first feature with each die in the set having a different technology node. A second set of die simulates a second feature with each die in the set having a different technology node. The die are inspected by the inspection tool and the technology node is determined for each feature at which no false defect inspection errors occur. The technology nodes for each feature at which no false defect reports occur is noted to define the runability of the inspection tool. In addition, one or more die having programmed defects may be inspected to determine the tool""s sensitivity, meaning the smallest defect that the inspection tool is able to detect.
The sets of features at different technology nodes are written on a photomask test plate as one or more runability modules, each module having an array of die. Each column of the array includes a set of die having a feature common to microelectronic devices. Each row of the array represents a technology node corresponding to industry technology nodes. Thus, for a given feature, inspection of die having that feature at different technology nodes allows a determination of the smallest technology node at which the inspection tool will successfully test for the feature without noting errors. Evaluation of the runability of a photomask inspection tool results from a determination of the minimum feature technology nodes for features relevant to the device being fabricated.
The present invention provides a number of important technical advantages. One important technical advantage is that a quantifiable evaluation of the robustness of a photomask inspection tool is made available. A determination is made of the smallest technology node that an inspection tool is able to detect without false detection of faults for a variety of device features. This enables device manufacturers to accurately access the technology node that an inspection tool can effectively inspect not only in light of the tool""s minimum detectable defect size but also for inspectability of specific features commonly found in industry design patterns. Thus, for instance, allocation of inspection tools to appropriate tasks and investment decisions for new inspection tools may be made based on runability rather than simply minimum detectable defect size. In addition, more accurate quantification of performance for new inspection tools and upgrades is made possible.
Another important technical advantage of the present invention is that it provides a quantifiable measurement of the runability of an inspection tool. A runability chart aids in the selection of inspection tools for particular inspection functions based on actual inspection results for known features and technology nodes. Also, runability charts aid in the verification of mask designs. The manufacturability and inspectability of new design patterns is evaluated by comparison of runability results for inspection tools with proposed designs, allowing design revisions that take advantage of existing inspection capability.